weather forecast

I thought this was interesting. Taken from here

Monthly forecasting
The weather beyond about a week ahead stretches even the most experienced weather forecaster. Complex numerical weather forecast models from the Met Office and the European Centre for Medium Range Weather Forecasting (ECMWF) are run many times for the month (and season) ahead to build up a picture of the likelihood of different weather types affecting the UK.

What do they mean?

Surely they can't mean it's too difficult for experienced forecasters with multi-million pound computer systems to forecast what the weather might be like, after all their climate models have predicted warmer weather for years to come - haven't they?